The present invention pertains to a technical field of seat weight measuring devices for measuring a seat weight including, for example, a load applied to a vehicle seat of an automobile. More specifically, the invention relates to the technical field of a seat weight measuring device having a sensor with measuring gauges such as strain gauges for measuring a force caused by the seat weight.
Japanese Unexamined Patent Application Publication No. 2000-258234 discloses a known seat weight measuring device, i.e., a device for receiving a force caused by the seat weight including a load applied to a vehicle seat of an automobile and having an arm for transmitting the force to a sensor. As illustrated in FIGS. 5A–5D, Japanese Unexamined Patent Application Publication No. 2000-258234 discloses a seat weight measuring device 100 which includes a pin bracket 101 for receiving the seat weight, an arm 102 for transmitting the force caused by the seat weight, a sensor 103 for measuring the seat weight by detecting the force transferred from tips 102A of the bifurcated arm 102, and a base frame 107 fixed to the vehicle body.
A stopper bolt 105 is passed through holes 106 on sidewalls 101L and 101R on the left and right sides of the pin bracket 101, respectively, and holes 109 formed on sidewalls 102L and 102R on the left and right sides of the arm 102, respectively. The stopper bolt 105 connects the pin bracket 101 and the arm 102 so that they are rotatable relative to each other. The stopper bolt 105 is passed through a vertically elongated hole 108 formed on sidewalls 107L and 107R on the left and right of the base frame 107, respectively.
The arm 102 is journalled with a pivot bolt 110 to the sidewalls 107L and 107R on the left and right side of the base frame 107, respectively. The tips 102A of the arm 102 transmit the force to upper and lower half arms 119 and 120 fixed with bolts 118 to force transmitters 104C and 104D on the ends of a sensor plate 104. In this way, the arm 102 receives the seat weight from the pin bracket 101, in the direction indicated by arrow a in FIG. 5(B), and transmits the force corresponding to the seat weight to the sensor plate 104 of the sensor 103 through the upper and lower half arms 119 and 120.
The sensor plate 104 of the sensor 103 is attached to a metal column 111 vertically disposed from the center of the bottom of the base frame 107 and is fixed by screwing a nut 112 with a metal washer 123 to the column 111. The sensor plate 104 flexibly bends in response to the force applied by the arm 102. Then, as illustrated in FIG. 6, the sensor 103 measures the seat weight by detecting the bending of the sensor plate 104 with four strain gauges 113, 114, 115, 116. The four strain gauges 113, 114, 115, 116 form a bridge circuit.
As illustrated in FIG. 6, the sensor plate 104 of the seat weight measuring device 100 has constrictions 104A and 104B. The four strain gauges 113, 114, 115, 116 are linearly disposed in the vicinity of these constrictions 104A and 104B so that the column 111 is disposed between the linearly arranged strain gauges 113, 114, 115, 116 with two strain gauges on each side. By disposing the strain gauges 113, 114, 115, 116 in the vicinity of the constrictions 104A and 104B of the sensor plate 104 in this way, when the force caused by the seat weight is transmitted to the force transmitters 104C and 104D of the sensor plate 104, one of the two strain gauges 113, 114 on one side of the column 111 and one of the two strain gauges 115, 116 on the other side of the column 111 are pulled in one direction while the other strain gauges are compressed in the other direction. Thus, one of the two strain gauges on the same side is pulled and the other is compressed. In this way, the sensitivity of the four strain gauges 113, 114, 115, and 116 forming the bridge circuit is increased and the force (i.e., the seat weight) can be measured accurately.
If the ambient temperature changes when the four strain gauges 113, 114, 115, 116 forming a bridge circuit measure the force transmitted to the sensor plate 104, the four strain gauges 113, 114, 115, 116 may become unbalanced and the accuracy of the measurement decreases. Therefore, to compensate for an error in the measurement caused by a change in the ambient temperature, a temperature sensing resistance 117 is disposed.
The sensor plate 104 has a layered structure with a lower insulating layer 126, a wiring layer 129, a resistive layer (which includes the strain gauges 113, 114, 115, 116) 130 and an upper insulating layer (a protective layer) 131 sequentially stacked on a metal substrate. Due to this structure, the heat resistance and corrosion resistance of the sensor plate 104 are improved.
In addition to the improvement of the heat resistance and corrosion resistance, the sensor plate 104 having such a layered structure may have an improved electromagnetic immunity (EMI) by adding a GND layer 127 on the metal substrate 125 with a lower insulating layer 126 interposed between the GND layer 127 and the metal substrate 125 (as shown, for example, in Japanese Unexamined Patent Application Publication No. 2003-097997). In such a case, the layered structure of the sensor plate 104 is as illustrated in FIG. 7.
More specifically, the sensor plate 104 is formed by stacking a lower insulating layer 126, a GND layer 127, a middle insulating layer 128, a wiring layer 129, a resistive layer (strain gauges) 130 and an upper insulating layer (protective layer) 131 sequentially on a metal substrate 125. However, it should be readily recognized that the sensor plate 104 shown in FIG. 7 is formed of many layers and, as a result, the production cost of this sensor plate 104 are high.
Moreover, if the ambient temperature of the seat weight measuring device 100 changes suddenly, a temperature difference for which the temperature sensing resistance 117 can not compensate may be generated between the resistances of the strain gauges 113, 114, 115, 116. Therefore, when such a sudden change occurs in the ambient temperature, there is a problem in that the force may not be measured accurately by the strain gauges 113, 114, 115, 116, even with a temperature sensing resistance 117. In such a case, a heat insulating material may be attached around the seat weight measuring device 100. The heat insulating material, however, cannot effectively suppress the change in the heat transmitted to the sensor plate 104 through the column 111.